Engineering plastics are those that can be molded into shapes such as gear wheels and can retain these shapes against impact and strain. In order to meet these requirements the plastic should not crystallize but remain amorphous. The development of crystallinity in the plastic reduces its resistance to impact. As a rule of thumb, clear plastics are amorphous, while opaque plastics contain a high degree of crystallinity. In addition to remaining amorphous, the plastics must retain their tensile strength to resist breaking under strain. Many plastics which when dry display high tensile strengths lose tensile strength by absorbing small amounts of moisture from the atmosphere, Ideally engineering plastics neither become crystalline nor are sensitive to moisture.
The homopolymers of the C.sub.19 diacid identified above and hexamethylene diamine are known to the art. It is also known that these homopolymers are sensitive to moisture and after absorbing as little as one half of 1 percent suffer drastic loss of tensile strength.
The homopolymers of isophthalic acid and terephthalic acids and hexamethylene diamine are known. These homopolymers reportedly become crystalline under certain conditions.